This invention relates to a method of forming an improved laminate and a laminated core formed therefrom. More particularly, the invention relates to applying a liquid bonding agent and a sealing agent to the facing surfaces of sheets and applying sufficient pressure to the sheets to remove excess liquid bonding agent to displace air from between the facing surfaces of the sheets while spreading the sealing agent to prevent reentry of the air and to prevent seepage of the liquid bonding agent along the edges of the sheets.
There are several applications such as electric power transformers, motors, electronics and catalytic converters employing thin gauge sheets. Thin gauge electrical steel sheets or amorphous metal sheets for electrical applications reduce magnetically induced eddy currents by reducing the cross-sectional area through which those currents may flow. Grain oriented steel sheets have a thickness less than 0.5 mm, typically in the range of 0.18 to 0.35 mm. Amorphous metal sheets typically have a thickness of about 0.02 to 0.05 mm.
It is well known the above type electrical devices are more efficient when the thickness of the sheet is decreased with the lower limit for the sheet thicknesses determined by manufacturing considerations. However, reducing the sheet thickness has undesirable effects on handling and fabrication productivity. Handling tissue-like thin sheets is a problem because the sheets are fragile and prone to damage during handling. The very thinness of the sheets reduces the productivity during processing and fabrication, making the product more labor intensive to utilize.
The prior art discloses adhesives, varnishes, oxides or mixtures thereof which may be applied to the surfaces of sheets so that several of the sheets can be bonded (or laminated) together for simultaneous processing. Processing such a laminate greatly increases productivity and diminishes handling problems since the laminate is thicker and more rigid than a single sheet.
Nevertheless, there are several disadvantages when using adhesives, varnishes or oxides to bond sheets which are discussed at length in pending U.S. patent application Ser. No. 4,882,834, filed Apr. 27, 1987, having a common assignee and incorporated herein by reference. For example, to develop a good bond between sheets requires the bonding agent to be applied as a relatively thick layer creating space between adjacent sheets. This is undesirable for bonded laminates used in electrical applications which are wound or stacked because the increased spacing between the sheets decreases the space factor. Even a thinly applied adhesive is undesirable because it tends to shrink when cured. Such a shrinkage, particularly for thin metal sheets, may strain or induce stress into the sheets. A further disadvantage when using chemical bonding is that an elevated temperature may be required to cure the bonding agent. Such an elevated temperature may diminish the effects of domain refinement treatments for electrical steel sheets. Another disadvantage when using chemical bonding is that the sheets become rigidly connected. Winding a rigidly formed electrical steel laminate into a coil may induce stress thereby increasing core losses of the laminate. A further disadvantage with chemical or ceramic bonding is that the bonding layers tend to be brittle. Cutting, punching or corrugating may fracture a brittle bonded layer causing the sheets to delaminate.
My U.S. Pat. No. 4,882,834 discloses a laminate can be formed that will resist separation indefinitely by applying a liquid of an appropriate viscosity to the facing surfaces of sheets and applying sufficient pressure to the sheets to remove excess liquid and to displace air from between the facing surfaces of the sheets. The liquid remaining between the facing surfaces forms a seal preventing reentry of the air which enables the laminate to resist separation during subsequent processing and fabrication. The laminate formed has no increase in the space between its sheets and no induced stress in its sheets. Unfortunately, some of the liquid remaining between the sheets seeps out from between the sheets along the sheet edges. Such seepage may cause a number of physical problems during subsequent processing of a laminate when punching to form stacked laminates. In the case where transformer oil was used as the bonding liquid to form laminated electrical steel sheets, oil buildup on a punch press caused the laminate to slip in the drive system, making it difficult to obtain precisely mitred cuts such as are needed to build a transformer core from electrical steel sheets. Seepage also may result in the bonding liquid transferring onto the exterior surfaces of the laminated sheets so that stacked cut laminated sheets become stuck together making it difficult to align properly in a core stack. Delamination may even occur when handling the laminated sheets or punchings.
Accordingly, there remains a need for an improved technique for forming a laminate using a liquid bonding agent wherein seepage of the liquid bonding agent from between the facing surfaces of the formed laminate is minimized or prevented.